This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Outdoor telecommunication equipments, such as Radio Remote Units (RRUs)/Radio Remote Heads (RRHs), Radio Base Stations (RBSs) and Base Transceiver Stations (BTSs), are often exposed to unexpected situations or occurrences, such as lightning strike or high voltage, which may generate huge energy during an extremely short period and cause system performance error or critical irreparable hardware damage. To protect the telecommunication equipments against such kind of damage caused by lightning strikes and electric surges, SPDs have been developed and applied.
Traditionally, it was believed that, to ensure design effectiveness for lightning strike protection, an SPD shall have multiple stages (two even three stages) to accommodate protective components. Between two consecutive stages, a serial inductor shall be connected, as illustrated in FIG. 1. In a case where the telecommunication equipment to be protected has large power consumption, the serial inductor may have a large dimension. In addition, in order to have enough inductance for the SPD, the inductor may increase its dimension and weight to avoid magnetic saturation in itself.
Due to the significant contribution of the inductor to the multi-stage SPD's dimension, little improvement has been achieved so far in reducing the SPD's dimension as illustrated in FIG. 2.
On the other hand, telecommunication equipments are increasingly subject to limitation on mechanical dimension, as also illustrated in FIG. 2. The unreduced dimension of the SPD constitutes a bottleneck for the miniaturization of the telecommunication equipments.
Considering that the large dimension of the multi-stage SPD is mainly due to the use of inductor, a single-stage SPD obtained by removing serial inductors as well as following stages from a multi-stage SPD has been proposed to fit for small telecommunication equipments. FIG. 3 depicts an example of such a single-stage SPD, wherein two protective components are contained in the only one stage of the SPD. Typically, the protective components include a Gas Discharge Tube (GDT) connected in series with a varistor.
In practical use, the single-stage SPD is connected in parallel with a load to be protected and clamps a load voltage Vload to a clamping voltage Vclamp not larger than a load voltage limit Vlimit when the load voltage Vload rises to a breakdown voltage VATH of the SPD.
Undesirably, the single-stage SPD has a variable breakdown voltage, which increases with a rising rate of a voltage across a load to be protected by the SPD. That is, the breakdown voltage VATH of the SPD is high when the load suffers from a steep voltage impulse and thus the load voltage Vload increases at a high rate as indicated by the dashed line in FIG. 4, while the breakdown voltage VATH of the SPD is low when the load suffers from a gentle voltage impulse and thus the load voltage Vload increases at a low rate as indicated by the solid line in FIG. 4.
Let VATH_DC and VATH_Imp@R respectively denote a Direct Current (DC) breakdown voltage corresponding to a gentle voltage increase and an impulse breakdown voltage corresponding to a steep voltage increase, a formula may be given as follows to describe the relationship between VATH_DC and VATH_Imp@R:VATH_Imp@R=α(R)×VATH_DC+β(R)  (1)where R represents the rising rate of the steep voltage increase, α(R) represents a function of R and takes a value higher than 1, and β(R) represents a function of R and takes a value higher than 0.
From the above formula, it can be concluded that VATH_Imp@R must be higher than VATH_DC. Accordingly, there is a risk for the impulse breakdown voltage VATH_Imp@R and hence the load voltage Vload to exceed the load voltage limit Vlimit and thus cause damage to the load, even if both the clamping voltage Vclamp and the DC breakdown voltage VATH_DC of the SPD are selected to be lower than the load voltage limit Vlimit.
For illustration, FIG. 5 depicts such a risky scenario where an SPD with a clamping voltage Vclamp≈40V and a DC break voltage VATH_DC=180V is used for protecting a telecommunication equipment with a load voltage limit Vlimit=200V. At the time point 1.0 us, a lightning strike with a peak current 1.8 KA and a 10/350 us waveform is injected into the telecommunication equipment and causes the load voltage Vload of the telecommunication equipment to increase at a relatively high rising rate R, which may result in an impulse breakdown voltage VATH_Imp up to 588V. In this case, the SPD is delayed to be triggered for clamping the load voltage Vload to the clamping voltage Vclamp until the time point 1.9 us when the load voltage Vload rises to 588V.
As a result, the load voltage Vload of the telecommunication equipment as illustrated by the solid line in FIG. 5 may exceed the load voltage limit Vlimit during the triggering delay Tdelay and cause irreparable damage to the telecommunication equipment.
According to FIG. 6 which illustrates how the peak voltage across the load of the telecommunication equipment and the triggering delay may change with the peak current of the lightning strike as a solid line and a dashed line respectively, the prior art single-stage SPD is incapable of protecting the telecommunication equipment against most lightning strikes in the natural world, whose peak currents are higher than 250 A and accordingly cause peak load voltages higher than the load voltage limit Vlimit=200V.
Simply selecting a DC breakdown voltage VATH_DC as low as possible cannot eliminate the risk for the impulse breakdown voltage VATH_Imp and hence the load voltage Vload to exceed the load voltage limit Vlimit, because the VATH_DC multiplied by the item α(R) in formula (1) and then added with β(R) may take a value larger than the load voltage limit Vlimit when the rising rate of the load voltage is high enough. Furthermore, when a VATH_DC less than 100V is selected for a DC system, it will interfere with normal DC voltages in the system, thereby causing a very low impedance and even a short circuit undesirably.
As a feasible solution for completely avoiding the risk for the impulse breakdown voltage VATH_Imp and hence the load voltage Vload to exceed the load voltage limit Vlimit, US2013/0114168A1 proposes a special SPD which is able to be triggered on/off by an external signal. Unfavorably, a complicated circuit needs to be designed for enabling communications between the SPD and an external circuit. In addition, being communicatively connected with the SPD, the external circuit itself is vulnerable to unexpected damage caused by high-level lightning strikes and hence unable to provide any external triggering signal to the SPD.